Carbon fiber-reinforced carbon composite material

ABSTRACT

A carbon fiber-reinforced carbon composite material having a porosity of not higher than 20% prepared by subjecting a carbon fiber-reinforced carbon composite material having a porosity of from 25 to 95% to densification treatment, wherein the densification treatment is conducted with pyrolytic carbon formed by the pyrolysis of a halogenated hydrocarbon-containing gas at a temperature of from 400° to 800° C.

This application is a continuation of application Ser. No. 07/068,167,filed on June 30, 1987, now abandoned.

The present invention relates to a carbon fiber-reinforced carboncomposite material (hereinafter referred to as "a C/C compositematerial") having excellent strength and frictional properties.

In general, a C/C composite material is produced by impregnating ormixing a thermosetting resin such as a phenol resin or a furan resin, ora thermoplastic resin such as a pitch material, orpolyacrylonitrile-type, pitch-type or rayon-type short or long carbonfibers, hot-pressing the impregnated or mixed material, followed bybaking at a temperature of from 600° to 2500° C. in a non-oxidizingatmosphere.

However, the C/C composite material obtained by this method has a highporosity since the carbonization yield of the matrix carbon is fromabout 40 to about 50%, and it has a drawback that the strength andfrictional properties are poor.

Therefore, in order to improve various properties of such conventionalC/C composite materials there have been proposed a method wherein aprocess of heat-treating the molded material, then impregnating a resinor a pitch material thereto and subjecting the impregnated materialagain to baking, is repeated to fill the resin or pitch material intothe pores for densification treatment, and a method wherein by means ofchemical vapor deposition, pyrolytic carbon formed at a high temperatureof at least 1000° C. from a hydrocarbon material, is filled in the poresof the C/C composite material for densification treatment. If necessary,subsequent to such an operation, a further heating is conducted toobtain a C/C composite material.

However, such conventional densification methods had drawbacks that inthe case of the impregnation method, the process was long andcumbersome, and in the case of the chemical vapor deposition method,heat treatment is conducted at a high temperature of at least 1000° C.,and the production rate of pyrolytic carbon is so small that a longperiod of time is required, which in turn leads to a high productioncost. The porosity of the C/C composite material obtained by theconventional densification treatment can be reduced to a level of 20% orlower, but such a C/C composite material is not necessarily satisfactorywith respect to the strength and frictional properties. Thus, a furtherimprovement has been desired.

Under the circumstances, the present inventors have conducted detailedstudies for the densification treatment of C/C composite materials by achemical vapor deposition method, and have previously proposed toimprove the frictional properties by providing a carbon coating layerpreferably having a thickness of at least 100 μm on the surface of a C/Ccomposite material by a chemical vapor deposition method using ahalogenated hydrocarbon-containing gas (Japanese Patent Application No.145988/1985). As a result of further studies, it has been found that asimilar technique is extremely effective also for the densification,whereby a C/C composite material having excellent strength andfrictional properties can be obtained at a low cost. The presentinvention is based on this discovery.

Namely, the object of the present invention is to provide a C/Ccomposite material having excellent properties obtained by densificationtreatment to bring the final porosity to a level of not higher than 20%.

This object can readily be accomplished by a carbon fiber-reinforcedcarbon composite material having a porosity of not higher than 20%prepared by subjecting a carbon fiber-reinforced carbon compositematerial having a porosity of from 25 to 95% to densification treatment,wherein the densification treatment is conducted with pyrolytic carbonformed by the pyrolysis of a halogenated hydrocarbon-containing gas at atemperature of from 400° to 800° C.

Now, the present invention will be described in detail with reference tothe preferred embodiments.

As the carbon fibers for the present invention, any known fibers such aspitch-type, polyacrylonitrile-type or rayon-type carbon fibers can beused. If necessary, inorganic fibers or inorganic materials made of e.g.silicon carbide, alumina or carbon black may be mixed.

Further, as to the shape, the carbon fibers may be used in the form of awoven fabric, a non-woven fabric or short fibers.

To such carbon fibers, a matrix material such as a phenol resin, a furanresin or a petroleum-type or coal-type pitch material, is mixed orimpregnated, followed by drying, to obtain a composition comprisingcarbon fibers and the matrix material. In this case, the matrix materialmay be used in the form of a solution having a proper viscosity, bydissolving such a material in a solvent such as an alcohol, acetone oranthracene oil.

Then, the composition comprising carbon fibers and the matrix materialis adjusted to bring the volume content of carbon fibers to a level offrom 10 to 70%, preferably from 20 to 50%, then filled in a mold andpress-molded at a temperature of from 100° to 500° C. Then, in an inertgas atmosphere such as nitrogen gas, the temperature is raised at atemperature rising rate of from 1° to 200° C./hr to a level of from 800°to 2000° C., followed by sintering, to obtain a C/C composite materialhaving a porosity of from 25 to 95%, preferably from 30 to 60%.

In the present invention, it is important to conduct densificationtreatment in such a manner that to this C/C composite material,pyrolytic carbon formed by thermally decomposing a halogenatedhydrocarbon as the starting material at a temperature of from 400° to800° C. usually for from 20 to 1000 hours, is applied so that thepyrolytic carbon fills the pores of the C/C composite material, toobtain a C/C composite material having a porosity of not higher than20%, preferably from 5 to 15%. The porosity of the resulting C/Ccomposite material can be controlled to a level of not higher than 20%by adjusting the conditions for the chemical vapor deposition reactione.g. by prolonging the reaction time, or by adjusting the deposition ofthe pyrolytic carbon, e.g. by lowering the concentration of the reactiongas. Further, depending upon the reaction condition, it may happen thatthe pyrolytic carbon covers the surface of the C/C composite materialand no further densification proceeds. In such a case, the densificationtreatment by the chemical vapor deposition should be stopped, and afterscraping off the surface covering layer of the C/C composite material,the densification treatment may be applied again by a chemical vapordeposition method.

As the halogenated hydrocarbon to be used in the present invention,various halogenated hydrocarbons including chlorinated hydrocarbons suchas dichloroethylene, dichloroethane and trichloroethane, may beemployed. Among them, chlorinated hydrocarbons wherein the halogen atomis a chlorine atom, and the molar ratio of the halogen atom to thehydrogen atom is equal, are preferred, since the deposition rate is highand it is thereby possible to reduce the production cost.

Further, the above-halogenated hydrocarbon is preferably used incombination with a carrier gas such as nitrogen, argon or hydrogen, andit is reacted with the C/C composite material heated to a level of from400° to 800° C. by e.g. a high frequency induction heating coil, toconduct densification treatment usually for a period of at least 20hours, as the case requires.

By the densification treatment under such reaction conditions, thepyrolytic carbon formed in the pores of the C/C composite material has alattice spacing of graphite crystal of at least 3.7 Å and a crystalthickness of at most 12 Å, as measured by an X-ray diffraction method.

This C/C composite material may be subjected to further heat treatment,as the case requires, whereby graphitization of the pyrolytic carbonfurther proceeds to form a graphite crystal structure having a latticespacing of not higher than 3.6 Å and a crystal thickness of at most 13Å. The C/C composite material having such a matrix layer will beexcellent in the strength and frictional properties.

Further, in the present invention, the densification treatment may beconducted solely by the densification treatment by the chemical vapordeposition using the halogenated hydrocarbon according to the presentinvention, or may be conducted by a combination of the method of thepresent invention with a conventional densification method by theimpregnation and baking of a pitch material or resin, or with adensification method by a conventional chemical vapor deposition methodusing a hydrocarbon as the starting material.

Namely, in the present invention, it is important that pyrolytic carbonhaving the structure specified by the X-ray diffraction method isprecipitated by a chemical vapor deposition method, and filled in thepores of the C/C composite material for the densification treatment ofthe C/C composite material.

According to the present invention, it is possible to produce a C/Ccomposite material having excellent strength and frictional propertiesat a low cost by a single process such that the densification treatmentof the C/C composite material is conducted by a chemical vapordeposition method by using a halogenated hydrocarbon.

Now, the present invention will be described in further detail withreference to Examples. However, it should be understood that the presentinvention is by no means restricted to such specific Examples.

EXAMPLE 1

21 parts by weight of pitch-type carbon fibers having a length of 10 mmwere mixed with about 79 parts by weight of an ethanol solution of aphenol resin, and the mixture was dried for one day and night. Themixture was then filled in a mold, and molded and cured at a temperatureof 250° C. to obtain a molded product having a carbon fiber content of50%.

This molded product was baked in a heating furnace in an argonatmosphere at 2000° C. to obtain a C/C composite material having aporosity of 48%.

The C/C composite material thus obtained was placed in an apparatus forchemical vapor deposition, and chemical vapor deposition treatment wasconducted by heating the composite material at 650° C. by a highfrequency induction heating, while supplying 13% by volume ofdichloroethylene vapor with argon gas as a carrier gas for 70 hours fromthe lower portion of the apparatus, whereby densification treatment wascarried out by filling the pyrolytic carbon in the pores of the C/Ccomposite material to obtain a C/C composite material having a porosityof 13%.

This C/C composite material was again subjected to heat treatment in anargon atmosphere at 2000° C. to obtain a C/C composite material of thepresent invention.

The C/C composite material thus obtained was machined into a disk form.The abrasion test was conducted by abrading a pair of disks to eachother. The results are shown in Table 1.

Further, the three-point flexural strength was measured. The results arealso shown in Table 1.

COMPARATIVE EXAMPLE 1

The C/C composite material having a porosity of 48% obtained in Example1 prior to the chemical vapor deposition treatment, was impregnated witha liquid pitch having a temperature of about 200° C., and then baked ina heating furnace at 800° C. in an argon atmosphere. Under the sametreating conditions, the impregnation and baking operation was repeatedeight times, so that the matrix of the carbonized pitch is filled in thepores of the C/C composite material for densification treatment, wherebya C/C composite material having a porosity of 13% was obtained. This C/Ccomposite material was subjected to heat treatment at 2000° C. in anargon atmosphere in the same manner as in Example 1, followed by thesame abrasion test and the three-point flexural strength test. Theresults are shown in Table 1.

EXAMPLE 2

A C/C composite material having porosity of 22% was obtained in the samemanner as in Example 1 except that the densification treatment wasconducted by filling pyrolytic carbon in the pores of the C/C compositematerial by a chemical vapor deposition treatment for 10 hours. Then,the same impregnation and baking operation as conducted in ComparativeExample 1, was repeated four times to fill the carbonized pitch matrixin the pores of the C/C composite material to obtain a C/C compositematerial of the present invention having a porosity of 13%.

This C/C composite material was subjected to heat treatment at 2000° C.in an argon atmosphere in the same manner as in Example 1, and then theabrasion test and the three point flexural test were conducted in thesame manner. The results are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                 Carbon                                                                            Pyrolytic carbon formed                                                                         Number of                                                                           Average                                     Flexural                                                                            fiber                                                                             from a halogenated                                                                        PV value*                                                                           abrading                                                                            friction                                    strength                                                                            content                                                                           hydrocarbon-containing                                                                    (kgm/cm.sup.2 ·                                                            operations                                                                          coefficient                                                                         Abrasion                              (kg/mm.sup.2)                                                                       (%) gas         sec)  (times)                                                                             (μ)                                                                              (mm/time)                      __________________________________________________________________________    Example 1                                                                            12    50  Present     80    100   0.28  0.001                          Example 2                                                                            11    50  Slighly present                                                                           80    100   0.27  0.001                          Comparative                                                                           7    50  Absent      80    100   0.50  0.040                          Example 1                                                                     __________________________________________________________________________     *PV value = Surface pressure P (kg/cm.sup.2) × circumferential          velocity V (m/sec)                                                       

We claim:
 1. A carbon fiber-reinforced carbon composite material havinga porosity of not higher than 20% prepared by subjecting a carbonfiber-reinforced carbon composite material having a porosity of from 25to 95% to densification treatment, wherein the densification treatmentis conducted with pyrolytic carbon formed by the pyrolysis of ahalogenated hydrocarbon-containing gas at a temperature of from 400° to800° C., wherein the pyrolytic carbon formed in the pores of thecomposite material subjected to the densification treatment has alattice spacing of graphite crystal of at least 3.7 Å and a crystalthickness of at most 12 Å, as measured by an X-ray diffraction method.2. The composite material according to claim 1, wherein thedensification treatment is conducted with pyrolytic carbon formed by thepyrolysis of a halogenated hydrocarbon-containing gas at a temperatureof from 400° to 800° C. and with carbon obtained by the impregnation andbaking of a pitch material or a resin.
 3. The composite materialaccording to claim 1, wherein the halogenated hydrocarbon is achlorinated hydrocarbon.
 4. The composite material according to claim 1,wherein the porosity of the carbon fiber-reinforced carbon compositematerial prior to the densification treatment is from 30 to 60%.
 5. Thecomposite material according to claim 1, wherein the porosity finallyobtained by the densification treatment is from 5 to 15%.
 6. Thecomposite material according to claim 1, wherein the halogenatedhydrocarbon is dichloroethylene, dichloroethane or trichloroethane. 7.The composite material according to claim 6, wherein the halogenatedhydrocarbon is dichloroethylene.